The present invention relates to a photovoltaic cell having a photoactive layer made of two components, namely an electron donor and an electron acceptor, particularly a conjugated polymer component and a fullerene component, and having two metallic electrodes provided on both sides of the photoactive layer.
Plastics having extensive xcfx80-electron systems, in which single and double bonds follow one another alternately in sequence, are referred to as conjugated plastics. These conjugated plastics have energy bands which are comparable with semiconductors in regard to electron energy, so that they may also be transferred from the non-conductive state into the metallically conductive state through doping. Examples of such conjugated plastics are polyphenylenes, polyvinylphenylenes (PPV), polythiophenes, or polyanilines. The efficiency of energy conversion of photovoltaic polymer cells made of a conjugated polymer is, however, typically between 10xe2x88x923 and 10xe2x88x922%. To improve this efficiency, heterogeneous layers made of two conjugated polymer components have already been suggested (U.S. Pat. No. 5,670,791 A), one polymer component being used as an electron donor and the other polymer component as an electron acceptor. By using fullerenes, particularly buckminsterfullerenes C60, as electron acceptors (U.S. Pat. No. 5,454,880 A), the charge carrier recombination otherwise typical in the photoactive layer may be largely avoided, which leads to a significant increase in efficiency. For reaching a good efficiency, good charge separation is a necessary, but not sufficient condition, because it must also be ensured that the separated charges also reach the corresponding electrodes of the photovoltaic cell. In typical photovoltaic cells of this type, a hole-collecting electrode made of indium/tin oxide (ITO) and an electrode-collecting electron made of aluminum have proven themselves.
The present invention is therefore based on the object of designing a photovoltaic cell of the type initially described in such a way that the charge transport between the photoactive layer and electrodes may be increased to elevate the short-circuit current.
The present invention achieves the object described in that an intermediate layer made of a conjugated polymer, which has a doping corresponding to the electrode potential and, in regard to the electron energy, has a band gap between the valence band and the conduction band of at least 1.8 eV, is provided between the photoactive layer and at least one electrode.
Since the conjugated polymer of the intermediate layer is doped according to the electrode potential, which means an oxidative doping in the region of the hole-collecting electrode and a reductive doping in the region of the electron-collecting electrode, the conjugated polymer ensures a hole excess in the region of the hole-collecting electrode, but an electron excess in the region of the electron-collecting electrode, so that, in the region of the oxidatively doped polymer, the hole conduction is reinforced, and the electron conduction is reinforced in the region of a reductively doped polymer. Since, however, the conjugated polymer of the respective intermediate layer has, in regard to the electron energy bands, a comparatively large band gap of at least 1.8 eV between the valence band and the conduction band, a correspondingly high activation energy results for the intrinsic conduction, which leads, in the case of an oxidatively doped polymer layer, to the electron conduction from the photoactive layer to the hole-collecting electrode being obstructed and, in the case of the reductively doped intermediate layer, to the hole conduction from the photoactive layer to the electron-collecting electrode being obstructed. With the aid of these separate intermediate layers, a valve effect may therefore be achieved, which supports the conduction of the charge carriers of the respective adjoining electrode which are to be collected from the photoactive layer to the electrode, but obstructs diffusion of opposing charges in the same direction. As a result of these separate layers, the charge conduction to the electrodes may correspondingly be improved, which directly results in an increase of the short-circuit current. It probably does not have to be emphasized that, depending on the application, photovoltaic cells may be used which have such an intermediate layer between the hole-collecting electrode and the photoactive layer, between the electron-collecting electrode and the photoactive layer, or in the region of both electrodes.
Although different conjugated polymers may be accordingly oxidatively or reductively doped to implement the intermediate layers, particularly advantageous ratios result if the intermediate layer is made of a doped polythiophene derivate. Both high polymers and oligomers are to be understood from the concept of polymer.